1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM), and more particularly to a magnetic random access memory which uses a magnetic tunnel junction (MTJ) which shows a tunnel magnetic resistance effect (TMR effect) as a memory cell.
2. Description of the Related Art
The resistance of a magnetic tunnel junction (MTJ) which is composed of two ferromagnetic layers and a tunnel barrier layer (tunnel insulating layer) put between these ferromagnetic layers changes largely, depending on directions of the magnetizations of the ferromagnetic layers. Such a phenomenon is called a tunnel magnetic resistance effect (TMR effect). Therefore, by detecting the resistance of the magnetic tunnel junction, it is possible to determine the direction of the magnetization of the ferromagnetic layer.
The magneto-resistance device containing the magnetic tunnel junction (MTJ) device is applied to a magnetic random access memory (MRAM) which can hold data without being erased. In such an MRAM, memory cells each containing the MTJ devices are arranged in a matrix. The direction of the magnetization of one of the two ferromagnetic layers contained in the magnetic tunnel junction device is fixed (the ferromagnetic layer is called a pinned ferromagnetic layer) and the direction of the magnetization of the other is freely reversible (the ferromagnetic layer is called a free ferromagnetic layer). Data is stored as the direction of the magnetization of the free ferromagnetic layer. A data write operation is carried out by applying a current to the neighborhood of the magnetic tunnel junction device and reversing the direction of the magnetization of the free ferromagnetic layer by the magnetic field generated by the current. A data read operation is carried out by detecting the direction of the magnetization of the free ferromagnetic layer by using the TMR effect.
In the MRAM, it is demanded that the data write operation, i.e., the reversal to the direction of the magnetization can be carried out in as small current as possible. On the other hand, in order to hold data stably, it is preferable that the direction of the magnetization of the free ferromagnetic layer is stable to thermal disturbance. However, generally, these are contradictory. For example, the direction of the magnetization can be reversed in the small current if the coercive of the free ferromagnetic layer is made small, i.e., an anisotropic magnetic field of the free ferromagnetic layer is made small. In this case, however, the decrease of the anisotropic magnetic field is generally accompanied by the reduction of an energy barrier to reverse the direction of the magnetization of the free ferromagnetic layer. Therefore, when the anisotropic magnetic field is decreased, the stability in the MRAM data holding characteristic is lost.
U.S. Pat. No. 6,396,735A as a first conventional example discloses a conventional MRAM in which the reduction of the write current and the stabilization of the data holding characteristic are achieved at a same time. As shown in FIG. 1, the conventional MRAM is composed of a magnetic memory device 101a. The magnetic memory device 101a is composed of a wiring layer 128, an insulating layer 127, an anti-ferromagnetic layer 111, a pinned ferromagnetic layer 112, an insulating layer 113, a free ferromagnetic layer 114, a wiring layer 115 and a ferromagnetic layer 116. The pinned ferromagnetic layer 112 is composed of ferromagnetic layers 120 and 122 and a metal layer 121 put between them. As shown in FIG. 1, the wiring layer 115 extends in parallel to the substrate.
In the MRAM shown in FIG. 1, the data write operation is carried out by applying a write current to a direction parallel to the substrate through the wiring layer 115. When the write current flows through the wiring layer 115, a magnetic field is generated and applied to the second ferromagnetic layer 114 and the third ferromagnetic layer 116, and the directions of the magnetizations of the free ferromagnetic layer 114 and the ferromagnetic layer 116 are reversed. Because the directions of the magnetic fields applied to the free ferromagnetic layer 114 and the ferromagnetic layer 116 are opposite to each other, the directions of the magnetizations of the free ferromagnetic layer 114 and the ferromagnetic layer 116 are opposite. Because the distance between the free ferromagnetic layer 114 or ferromagnetic layer 116 and the wiring layer 115 is short, it is possible to reverse the direction of the magnetization in few currents.
On the other hand, because the free ferromagnetic layer 114 and the ferromagnetic layer 116 are magneto-statically combined and the directions of the magnetizations are opposite when a write current does not flows, the directions of the magnetizations of the free ferromagnetic layer 114 and the ferromagnetic layer 116S are stabilized to external disturbance. A technique similar to the first conventional example is disclosed in U.S. Pat. No. 6,252,796A.
When the data write operation is carried out by applying a current to the wiring layer 115 put in between the free ferromagnetic layer 114 and the ferromagnetic layer 116, how the wiring layer 115 is connected with a wiring line for supplying the write current is a problem. For solving this problem, it is necessary to provide the free ferromagnetic layer 114 near to the ferromagnetic layer 116, in order to magneto-statically combine the free ferromagnetic layer 114 and the ferromagnetic layer 116. Therefore, it is necessary to pattern the free ferromagnetic layer 114 and the ferromagnetic layer 116 so that the ends of them are approximately aligned. In this case, the wiring line for supplying the write current must be drawn out outside the free ferromagnetic layer 114 and the ferromagnetic layer 116 to connect with the wiring layer 115. Thus, conventionally, it is necessary to pattern the free ferromagnetic layer 114, the ferromagnetic layer 116 and the wiring layer 115 as a non-magnetic conductive layer separately, resulting in increase of the number of processes.
Also, there is a case that the ends of the free ferromagnetic layer 114 and the ferromagnetic layer 116 intersect due to an alignment difference. In such a case, the reversal of the direction of the magnetization cannot be smoothly carried out.
For these reasons, it could be considered that the ferromagnetic material 116 is patterned and then is embedded by an oxide film 124 for flattening, as shown in FIG. 2. By using this method, the end of the free ferromagnetic material 114 does not cover the end of the ferromagnetic material 116, because the ferromagnetic material 116 is embedded in the oxide film 124, even if the alignment difference is caused.
However, in this method, the flattening process gives damage to the ferromagnetic film 116. Moreover, it is necessary to provide a flattening stopper layer on the ferromagnetic layer 116. For this reason, if the distance between the free ferromagnetic layer 114 and the ferromagnetic layer 116 is made large, the magneto-statical combination and the exchange combination are weakened. As a result, the stability in the directions of the magnetizations of the free ferromagnetic layer 114 and the ferromagnetic layer 116 is lost in the non-write operation. Moreover, a manufacturing cost increases through the increase of the number of processes by the necessity of the flattening process.
Moreover, the alignment difference is caused through individual patterning of the free ferromagnetic layer 114 and the ferromagnetic layer 116. As a result, the magneto-statical combination between them is weakened and the reverse magnetic field of the free ferromagnetic layer 114 is deviated. Therefore, the value of the write current is deviated for every device.
In conjunction with the above description, a method of manufacturing a magnetic resistance effect head is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-255905). In this conventional example, an anti-ferromagnetic layer or a ferromagnetic layer is provided to direct contact a ferromagnetic magnetic resistance effect layer in order to generate a vertical bias magnetic field by exchange combination of the ferromagnetic magnetic resistance effect layer and the anti-ferromagnetic layer or ferromagnetic layer. Ion implantation is carried out a part of the anti-ferromagnetic layer or ferromagnetic layer.
Also, a magnetic resistance effect device is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 10-255231). The magnetic resistance effect device of this conventional example is composed of a ferromagnetic tunnel junction section and magnetic domain control films. The ferromagnetic tunnel junction section is a laminate film of a first ferromagnetic film, an insulating film and a second ferromagnetic film. The magnetic domain control films are provided at ends of one of the first and second ferromagnetic films.
Also, a magnetic tunnel junction device is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-161919). The magnetic tunnel junction device of this conventional example is composed of a pinned ferromagnetic multiplayer, a free ferromagnetic multiplayer, and an insulting tunnel layer. The pinned ferromagnetic multiplayer is composed of first and second ferromagnetic films and an anti-ferromagnetic film provided between the first and second ferromagnetic films to contact them. The first and second ferromagnetic films are anti-ferromagnetically combined with each other and have a magnetic moment fixed in a direction under an applied magnetic field. The free ferromagnetic multiplayer is composed of first and second ferromagnetic films and an anti-ferromagnetic film provided between the first and second ferromagnetic films to contact them. The first and second ferromagnetic films are anti-ferromagnetically combined with each other and have a magnetic moment reversible under an applied magnetic field. The insulting tunnel layer is provided between the pinned ferromagnetic multiplayer and the free ferromagnetic multiplayer to contact them, and allows a tunnel current between the pinned ferromagnetic multiplayer and the free ferromagnetic multiplayer.
Also, a magnetic resistance effect head is disclosed in Japanese Laid Open Patent Application (JP-P2001-52316A). The magnetic resistance effect head of this conventional example uses a ferromagnetic tunnel combination film which includes a free layer, a pinned layer, and a barrier layer formed between the free layer and the pinned layer. Oxide or nitride of metal material of the ferromagnetic tunnel combination film exists in a pattern of the he ferromagnetic tunnel combination film.
In addition, a magnetic resistance effect device is disclosed in Japanese Laid Open Patent Application (JP-P2002-176211A). The magnetic resistance effect device of this conventional example includes first and second magnetic layers, and a non-magnetic layer formed between the first and second magnetic layers. A current for sensing change in resistance based on change in an angle between a magnetization direction of the first magnetic layer and a magnetization direction of the second magnetic layer flows in a direction perpendicular to the surface of the each layer. The area of the non-magnetic layer is equal to or less than 1 μm2. One selected from the first and second magnetic layers and the non-magnetic layer has a first region and a second region, and the area of the first region is smaller than the area of the non-magnetic layer. The current flows through the first region, and the second region is formed of oxide, nitride or oxidized nitride of material of the first region.